Remove slug from custom post type 404

I have created a new page type and would like to remove the first part of the url so that all of url’s are in the format /%postname%/

The “Remove slug from custom post type” looks like it’s exactly what I need but I can’t get it to work. I just get a 404 when I go to view the page.

I think the problem is to do with what it’s adding to my .htaccess – it seems to think my custom page type is called ‘book’. Perhaps this is a default and I’m meant to edit it but I wouldn’t have thought it would be too hard for it to detect custom post types installed so perhaps it’s detecting it wrong? When I correct it manually, I still get a 404.

I have found others with the same problem but the solution don’t work for me – I do not have flush_rewrite_rules() anywhere in my functions.php and I have been to Settings > Permalinks and clicked submit with it set to /%postname%/. I also tried deactivating/reactivating the plugin – still the same.

Any help would be greatly appreciated!